1. Field of the Invention
The present invention relates to an injection molding machine and, more particularly, to an injection pressure control apparatus for an injection molding machine using a servo motor as a drive source of an injection mechanism.
2. Description of the Related Art
An injection molding machine in which an injection mechanism is driven by oil pressure to axially move a screw to perform injection, whereby injection pressure is controlled by hydraulic control, is known.
On the other hand, an injection molding machine in which an injection mechanism is driven by a servo motor to perform injection has also been developed. In order to control an injection pressure in an injection molding machine driven by a servo motor to perform injection, an output torque of the servo motor must be controlled. One of methods of controlling a torque of a servo motor is to perform torque limiting. In general, when torque limiting is performed during rotation of a servo motor, it is also performed during acceleration and deceleration. During acceleration, no inconvenience occurs because speed increases slowly. However, during deceleration, a sufficient damping force may not be applied to the servo motor due to such torque limiting, and an operation portion of an injection molding machine, such as a screw, may overrun by its inertia or the like. Especially when injection of a screw without molding material is performed in purging or the like, since a reaction force from the molding material is not applied to the screw unlike the case in which injection molding is performed by actually injecting a molding material to an injection mold, the screw particularly tends to overrun. Upon overrun of the screw, the distal end of the screw strikes against a heating cylinder, damaging the screw, the heating cylinder, a nozzle or the like.